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Disinfection methods can be broadly classified into physical methods and chemical methods.
Physical methods mainly include mechanical filtration, heating, freezing, radiation, micro-electrolysis, ultraviolet and microwave sterilization;
The chemical methods mainly include chlorine, chlorine dioxide, ozone, chloramine, halogen, metal ions, anionic surfactants and other biocides. The research and application of chlorine, ozone, chlorine dioxide and chloramine in chemical disinfection methods are numerous. In recent years, due to the increase in reports on chemical disinfection by-products and the increasing requirements of water quality standards, physical disinfection methods, especially ultraviolet rays. Disinfection has caused great attention from professionals.
Chlorine disinfection
When chlorine reacts with water, it usually produces a "disproportionation reaction" to produce hypochlorous acid (HOCL) and hydrochloric acid (HCL). The reaction equation is: 
Chlorine is mainly sterilized by hypochlorous acid because it is a small, neutral molecule that diffuses to the surface of a negatively charged bacteria and has a strong penetrating power that penetrates the cell wall and enters the interior of the bacteria. The effect of chlorine on bacteria is to destroy its enzyme system, causing bacterial death. The effect of chlorine on the virus is mainly the lethal effect on nucleic acid destruction.
Since the beginning of the twentieth century, the chlorination process has been widely used in water disinfection processes. At present, chlorination disinfection is still the application of Zuiguang's chemical disinfection method. Its main features are: (1) when the amount of treated water is large, the treatment cost per unit of water is low; (2) the water body can be kept for a long time after chlorine disinfection A certain amount of residual chlorine, thus having a continuous disinfection capacity; (3) a long history of chlorine disinfection, more experience, is a relatively mature disinfection method.
However, since 1974, Luke and Boer detected organic compounds such as chloroform and trihalomethanes (THMs) in urban tap waters in the Netherlands and the United States. In 1976, the National Cancer Institute of the United States conducted oral chloroform experiments on rats and mice. To determine that it is a carcinogen, it has been found that after disinfection of drinking water, the water contains harmful disinfection by-products such as teratogenic, carcinogenic, and mutagenic THMs. With the deepening of the research on the harmfulness of THMs, research on other disinfection by-products has been caused. There are more than 500 disinfection by-products known to date, but most of them are only in the microgram/liter (μg/L) grade, and many disinfection by-products have not been further studied. Among the large number of disinfection by-products, there are only more than 20 kinds of trihalomethanes, haloacetic acids, haloacetonitriles, halogenated ketones, halogenated aldehydes, halogenated phenols, etc., among which there is consensus on the carcinogenicity of THMs. Most of them are generally toxic and partly have a sudden onset. The production of halogenated organic compounds such as THMs is mainly the result of the action of organic matter and chlorine in water, while urban domestic sewage contains a large amount of organic matter. After chlorine disinfection, it will produce disinfection by-products such as halogenated organic matter, and enter the surface water body with sewage, polluting water source. And it has a toxic effect on aquatic organisms such as fish.
In order to avoid the generation of harmful disinfection by-products, the main ways are: (1) pretreatment to remove trihalomethane precursors (mainly fulvic acid and humic acid); (2) using alternative disinfectants or disinfection methods, in recent years A large amount of research has been conducted on the use of ozone, chlorine dioxide and chloramine instead of chlorine as a disinfectant.
2. Ozone disinfection Ozone is a strong oxidant. Ozonation is the same as chlorination. It acts both as a disinfectant and as an oxidizing agent. However, ozone is more sterilizing and oxidizing than chlorine. It can oxidize organic matter in water and kill it. Viruses, spores and bacteria. Ozone is produced in the field by air or pure oxygen through an ozone generator with yields of 1%-3% and 2%-6%, respectively.
The history of ozone as a disinfectant is almost as long as chlorine. In 1906, the water plant in Nice, France, used ozone to disinfect drinking water for the first time. In the early 1970s, American engineers began to use ozone instead of chlorine to disinfect sewage. According to the current research, it can be found that: (1) ozone disinfection reaction is rapid, sterilization efficiency is high, and at the same time, residual organic matter, color, smell, taste, etc. in water can be effectively removed, and the influence of pH value and temperature is small. (2) Ozone can reduce the amount of halogenated alkane disinfection by-products such as THMs in water. (3) Ozone disinfection can reduce the concentration of total organic halides in water.
Although ozone disinfection itself does not produce alkyl halides and total organic halides, other disinfection by-products such as aldehydes, ketones, alcohols, etc., which are formed, are chlorinated to produce trihalomethanes. According to reports, there are 2221 organic compounds detected in various water bodies in the world. Ozone can react with various organic substances to form a series of intermediate products, which can be roughly divided into organic by-products and inorganic by-products. Organic by-products are represented by formaldehyde, and it has been reported that formaldehyde is a carcinogen. The inorganic by-product of concern to zui is bromate, and the International Agency for Research on Cancer (IARC) classifies bromate as carcinogenic 2B, a possible carcinogen. Because ozone has very low solubility in water, is easily decomposed, has poor stability, and has almost no residual disinfecting ability, ozone is commonly used in combination with other disinfectants as a preferred method for controlling harmful disinfection by-products such as THMs. According to reports in 1982, there are more than 1,100 water plants in the world that use ozonation. Among them, ozone is the only disinfectant. Except for a few in Europe, there is only one in the United States and Canada, and the others are supplemented with chlorine or chlorine. Amine disinfection to ensure the remaining disinfectant in the water. In addition, due to the poor stability of ozone, it is easy to be decomposed into oxygen, so it cannot be stored and transported in bottles. It must be used on site for timely use, with large equipment investment, high power consumption and high cost; operation management is complicated.
3. Chlorine Dioxide Disinfection Chlorine dioxide is also a strong oxidant. Its oxidizing ability is 25 times that of chlorine. Its disinfecting ability is second only to ozone and higher than chlorine. In 1944, the Niagara Falls Waterworks in the United States succeeded in using chlorine dioxide to disinfect drinking water in order to eliminate the odor generated by algae reproduction. In the 1970s, it was gradually used as a commonly used disinfectant. In many countries in Europe and America, chlorine dioxide was used in various water treatments. Tests have shown that chlorine dioxide is superior to chlorine in controlling the formation of THMs and reducing total organic halogens. Chlorine dioxide and humic acids such as humic acid and fulvic acid in water do not produce THMs, even in drinking water. In the process, the addition of a small amount of chlorine dioxide can also effectively inhibit the formation of THMs. Chlorine dioxide is a broad-spectrum disinfectant that has high inactivation effect on pathogenic microorganisms including viruses, spores, fungi, pathogenic bacteria and botulinum in water, with residual disinfection capacity, chlorine dioxide against spores and viruses. The inactivation effect is more effective than chlorine, and the sterilization effect is not affected in high pH and ammonia-containing water. In addition, chlorine dioxide has a strong ability to remove color, smell and taste in water.
The starting materials for the preparation of chlorine dioxide are sodium chlorate and sodium chlorite, depending on the amount of chlorine dioxide used. In the field of water treatment, the amount of chlorine dioxide used is generally small, and it is generally prepared by reacting sodium chlorite with chlorine. The reaction formula is: 
Because sodium chlorite cannot be stored, it must be prepared on site for timely use, and sodium chlorite is expensive and costly. When the reaction is incomplete, the free chlorine also reacts with the organic matter, possibly generating THMs. 50 to 70% of chlorine dioxide added to water is converted to ClO2- and ClO3-. Many experiments have shown that ClO2- and ClO3- are harmful to red blood cells, which can cause methemoglobinemia, which may interfere with the absorption and metabolism of iodine. Increase blood cholesterol.
4. Chloramine disinfection Chloramine disinfection has the following three advantages over chlorine disinfection: (1) reduce the production of THMs during disinfection; (2) can maintain a long time, can effectively control the residual bacteria in the water; (3) Avoid the odor generated when the free residual chlorine is too high. Chloramine disinfection is usually done by first adding ammonia, and then adding chlorine after adding it. If ammonia is added after chlorination, it is difficult to control the concentration of THMs. In addition, if ammonia is added for a long time after adding chlorine, it will become the main disinfectant with free residual chlorine, and chloramine is the auxiliary disinfectant. The disadvantage of chloramine disinfection is that it requires a longer contact time; the operation is complicated by the need to add ammonia. Chloramine has a poor bactericidal effect and should not be used as a disinfectant for drinking water alone. However, if it is combined with chlorine, it can not only ensure the disinfection effect, but also reduce the production of trihalomethanes, and can extend the action time in the water distribution network.
Among the four disinfectants commonly used in the above water treatment, the bactericidal ability of ozone is high, but the ozone itself is easily decomposed and the disinfection has no persistence; the chlorine dioxide has both a strong bactericidal ability and a relatively good durability; Chlorine has strong inactivation ability against bacteria, but it has poor ability to inactivate viruses and has no inactivation ability to spores. Although chloramine is strong in persistence, its bactericidal effect is not as good as chlorine, and it is generally not a single disinfectant. Studies have shown that the priority of inactivation efficiency of the four disinfectants at pH 6-9 is: ozone > chlorine dioxide > chlorine > chloramine; and the priority of stability is: chloramine > chlorine dioxide > chlorine > ozone.
5. UV disinfection Although traditional chemical disinfection methods are widely used in water supply and sewage treatment, people who are in the water treatment industry focus their attention on the fact that chemical disinfectants are added to the water to produce harmful disinfection by-products. To the ultraviolet disinfection method.
According to the biological effect, the ultraviolet light is divided into four parts according to the wavelength: A-band (UV-A), also known as black spot effect ultraviolet light, the wavelength range is 400nm~320nm; B-band (UV-B), also known as The erythema effect ultraviolet light, the wavelength range is 320nm~275nm; the C-band (UV-C), also known as sterilized ultraviolet light, the wavelength range is 275nm~200nm; the D-band (UV-D), also known as vacuum ultraviolet light, the wavelength range is 200nm~10nm. Water disinfection mainly uses C-band ultraviolet light, that is, C-band ultraviolet light can deactivate the DNA of bacteria, viruses, spores and other pathogenic bacteria, thereby destroying their ability to replicate and spread diseases.
A large number of studies and experiments have proved that ultraviolet disinfection of water is mainly through ultraviolet radiation to microorganisms. The nucleic acid in the living body absorbs the ultraviolet light energy, and damages and destroys the function of nucleic acid to cause the microorganism to die, thereby achieving the purpose of disinfection. . Life science reveals that nucleic acid is the basic material and life foundation of all living things. A nucleic acid is a biopolymer compound that is made up of many different nucleotides linked by a phosphodiester bond. Nucleic acids are classified into two major categories, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), depending on the composition. The common point is that the polynucleotide chain is linked by the principle of phosphodiester bond pairing with pyrimidine base pairing. . Nucleic acids are present in the cells of all living things and play a decisive role in the life processes of biology, metabolism, and variation. Microorganisms are exposed to ultraviolet radiation and absorb the energy of ultraviolet light. In fact, nucleic acids absorb the energy of ultraviolet light. The absorption spectrum of ultraviolet light by DNA and RNA ranges from 240 nm to 280 nm, and the absorption at a wavelength of 260 nm reaches a large value of zui. Ultraviolet light can alter nitrogen-containing heterocycles in DNA and RNA to cause the formation of new bonding molecules. The action of ultraviolet light on nucleic acids can lead to breakage of bonds and chains, cross-linking between strands, and formation of actinic products. The formation of dimers destroys the normal pairing of pyrimidines and purines, changes the biological activity of DNA, and prevents the microbes from replicating themselves. This is an important UV damage and a fatal damage to microbes. Numerous studies have confirmed that the increase in pyrimidine dimer is directly proportional to bacterial mortality.
Most UV devices use traditional low-pressure UV lamp technology, and some large water plants use low-pressure high-intensity UV lamp systems and medium-pressure high-intensity UV lamp systems. The high-intensity ultraviolet rays may reduce the number of lamps by more than 90%. The footprint is reduced, installation and maintenance costs are saved, and UV disinfection is also applicable to poor water quality. 
Regardless of which UV lamp is based on the same physical phenomenon, electromagnetic radiation is emitted when discharged from the mercury plasma region of the fluorescent lamp.
UV disinfection has many advantages that cannot be replaced by chemical methods: in some industries, such as aquaculture and food industry, there is no need for the persistence of chemical disinfectants, otherwise the aquatic organisms will die due to the influence of chemical agents, and the smell will be produced in food. Such side effects, and chlorination will form harmful disinfection by-products such as trihalomethanes; in some biotechnology such as fermentation, it is necessary to disinfect the water and inoculate the bacteria required for the process, so that the continuous disinfection effect is obviously unnecessary. In circulating water systems, frequent use of chlorine disinfection can cause corrosion problems, such as swimming pools, as well as groundwater recharge in oil extraction. If chemical disinfection is used, bacteria tend to develop resistance and continue to multiply in the soil to block The formation forms secondary pollution; the disinfection speed is fast, the efficiency is high, the floor space is small; the equipment is simple to operate, and it is easy to operate and realize automation. In recent years, the ultraviolet disinfection equipment used for water treatment has been widely used. UV-sterilized sewage can be reused in many areas to achieve wastewater recycling. It can be used to irrigate farmland, woodland and lawn to avoid damage to plants by chemical disinfectants; groundwater recharge can prevent formation blockage caused by re-propagation of microorganisms to chemical disinfectants.
7. Conclusions Applications in UV water treatment have been around for decades. However, due to its complicated technology and high cost, its application is limited. In the 1970s, due to increased water pollution and increased public health awareness, people were forced to adopt new methods based on traditional water treatment processes to ensure that water quality meets safer drinking water standards. After nearly 20 years of research and practice, the composite application technology based on ultraviolet light has become the first choice for deep purification technology of water supply with its good treatment effect.
(1) UV light used in the secondary treatment of urban sewage can be disinfected to meet the current domestic landscape and greening water requirements.
(2) The technology has the characteristics of no secondary pollution and has broad application prospects.
(3) Low energy consumption, low operating cost; high degree of automation; easy maintenance.
(4) In terms of equipment selection, the comprehensive technical and economic indicators of low-pressure high-intensity ultraviolet lamps are superior to medium-pressure high-intensity ultraviolet lamps.
With the in-depth study of the ultraviolet disinfection mechanism, the continuous development of ultraviolet technology and the continuous improvement of the design of the disinfection device, the ultraviolet disinfection method is expected to become one of the main methods to replace the traditional water disinfection. (Contributed by: Wuxi Jiangda Liansheng Water Treatment Technology Co., Ltd.)
Comparison of UV disinfection and four chemical disinfection methods
At present, the water supply and drainage disinfection methods can be divided into two categories, namely chemical disinfection and physical disinfection. Chemical disinfection methods include chlorination and ozone disinfection; physical disinfection methods include ultraviolet disinfection. Applications in UV water treatment have been around for decades. However, due to its complicated technology and high cost, its application is limited. In the 1970s, due to increased water pollution and increased public health awareness, people were forced to adopt new methods based on traditional water treatment processes to ensure that water quality meets safer drinking water standards. After nearly 20 years of research and practice, the composite application technology based on ultraviolet light has become the first choice for deep purification technology of water supply with its good treatment effect. The following section is selected from "Review: UV disinfection technology, advantages and prospects", the author of Wuxi Jiangda Liansheng Water Treatment Technology Co., Ltd.